Global warming: New Arctic forests could spur net carbon gain

SUMMIT COUNTY — While some climate research suggests that warming temperatures could lead to faster tree growth, and thus, more carbon capture, new research from the University of Exeter concludes that new forests growing in the Arctic could result in a net gain of carbon dioxide in the atmosphere.

That’s because those new forests will free up carbon previously held frozen in the tundra and release it as part of their growth cycle, the researcher said. Those emissions could outweigh increased carbon sequestration by forests in other parts of the world.

As a basis for their research, the scientists acknowleged that the Arctic is getting greener as plant growth increases in response to a warmer climate. The new study, published in Nature Climate Change, shows that, by stimulating decomposition rates in soils, the expansion of forest into tundra in arctic Sweden could result in the release of carbon dioxide to the atmosphere.

“Determining directly how carbon storage is changing in high-latitude ecosystems is very difficult because the majority of the carbon present is stored below ground in the soils,” said University of Exeter geographer Dr. Iain Hartley, lead author of the paper.

“Our work indicates that greater plant biomass may not always translate into greater carbon storage at the ecosystem level. We need to better understand how the anticipated changes in the distribution of different plant communities in the Arctic affects the decomposition of the large carbon stocks in tundra soils if we are to be able to predict how arctic greening will affect carbon dioxide uptake or release in the future,” he said.

By measuring carbon stocks in vegetation and soils between tundra and neighbouring birch forest, it was shown that, compared to tundra, the two-fold greater carbon storage in plant biomass in the forest was more than outweighed by the smaller carbon stocks in forest soils.

“It shows that the encroachment of trees onto Arctic tundra caused by the warming may cause large release of carbon to the atmosphere, which would be bad for global warming,” said Dr, Gareth Phoenix, of the University of Sheffield’s Department Animal and Plant Sciences.

“This is because tundra soil contains a lot of stored organic matter, due to slow decomposition, but the trees stimulate the decomposition of this material. So, where before we thought trees moving onto tundra would increase carbon storage it seems the opposite may be true. So, more bad news for climate change,” Phoenix said.

The results of the study are in sharp contrast to the predictions of models which expect total carbon storage to increase with the greater plant growth. Rather, this research suggests that colonisation by productive, high-biomass, plant communities in the Arctic may not always result in greater capture of carbon dioxide, but instead net losses of carbon are possible if the decomposition of the large carbon stocks in Arctic soils are stimulated.

This is important as Arctic soils currently store more carbon than is present in the atmosphere as carbon dioxide and thus have considerable potential to affect rates of climate change. It is yet to be seen whether this observed pattern is confined to certain soil conditions and colonising tree species, or whether the carbon stocks in the soils of other arctic or alpine ecosystems may be vulnerable to colonization by new plant communities as the climate continues to warm.